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Methodology IV: GWS® MICRO by Al-Pro


GWS® MICRO

Application

Model resolutions of less than 1 km are necessary for detailed analysis of localized projects. To be able to refine the resolution to less than 1 km, further wind flow models have to be applied. Mesoscale models usually do not achieve model resolutions of a few hundred meters. AL-PRO has developed a model system named GWS® MICRO that couples the results of the GWS® MESO modeling to the wind flow fields of a CFD model, achieving grid resolutions of 200 x 200 m by default. GWS® MICRO wind maps are especially suited for a refined site selection within an area of approximately 20 x 20 km (up to 30 x 30 km). GWS® MICRO wind maps support the developer with the first assessment of the yield potential of a project area and are especially suited for locating appropriate sites to erect met masts. Based on a GWS® MICRO map, one can narrow down site exploration to a few hundred meters. A site inspection is recommended to verify suitable locations for met masts.

Meteodyn WT and
AnemoScope

 andAnemoScope

Depending on the topographical characteristics of the model area, two different micro models are used for the microscale computations: The Meteodyn WT CFD model is used in areas with steep and complex topography [14] and MS-Micro/3 of Environment Canada [15] is used in areas with simple to moderate terrain slopes. Both methods use the mesoscale wind information to initialize the microscale simulations.

3D CFD

Meteodyn WT [2] is a French software package for wind energy assessment, based on 3D Computational Fluid Dynamics (CFD) simulations of the wind flow over given terrains. The CFD simulations are based on the integration of non-linear Reynolds Averaged Navier-Stokes (RANS) equations over a portion of the lower atmosphere, the computational domain. Turbulence is taken into account using the standard k-ε turbulence model. Meteodyn WT is therefore able to assess wind resources with a higher degree of accuracy than other models that utilize linearized equations. Even terrain with fairly complex features can be processed with Meteodyn WT.

MS-Micro/3

MS-Micro/3 is a component of AnemoScope and simulates 3D steady-state surface-layer flow, accounting for detailed spatial variations in terrain height and surface roughness. It solves linearized equations of motion using a discrete Fourier transformation. High spatial resolution, in all three dimensions, is achieved at low computing cost.

Geophysical data
SRTM and
CORINE

The SRTM data [16] from the USGS data base [17] is used to model the topography. The roughness field is determined from the CORINE [18] or GLC [9] land use data. Both data packages are interpolated to the model resolution.

Enhancement of
modeling results

The quality of the computational results of the GWS® MICRO modeling with Meteodyn WT can further be enhanced using representative wind data of met stations located in the region of interest. The wind measurements should comply with the standards used in the wind industry and ideally last at a minimum for 12 months. The data itself should be edited according to standard technical directives like the German directive „Technische Richtlinie Teil 6/Rev. 7” [19].

Resolution and
map size

Al-Pro uses a resolution of 200 x 200 m within Meteodyn WT to calculate the final model. This fine resolution limits the model area to 30 x 30 km.

Limitations

The computation results of the GWS® MESO modeling serve as input for the GWS® MICRO modeling with Meteodyn WT. That implies the existence of a Meso map covering the region of interest as precondition to the micro scale computations. The correspondent GWS® MESO map(s) has(have) therefore to be computed first. GWS® MICRO wind maps do not in the least replace a detailed wind study complying with current guidelines and regulations [20] and do not nearly achieve the necessary accuracy.
The accuracy of the GWS® MICRO simulations depends significantly on the complexity and kind of the terrain and land use and on the quality and representativity of the wind measurements. Especially in areas with steep slopes close to peaks and mountain ridges GWS® MICRO can overestimate the wind quantity. Such areas on the other side usually are not included in wind farm planning due to topographical characteristics. As a rule of thumb, the accuracy of the GWS® MICRO simulations at a resolution of 200 x 200 m range between +/- 1 m/s with regard to the mean annual wind velocity. With appropriate wind measurement data, accuracy can improve to +/- 0.5 m/s.

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